Electrographic marking upon an image recording medium comprises a two-stage process. First, air irons are created and charged ions of a given sign (usually negative) are deposited at selected image pixel locations to form an electrostatic charge on a recording medium. Then, the electrostatic charge image is made visible by "toning", which usually involves the passing of the recording medium, bearing the latent non-visible) image, into contact with a liquid solution containing positively charged dye particles in a colloidal suspension. The dye particles will be attracted to the negative charge pattern and the density of the dyed image will be proportional to the potential or charge on the medium.
Two types of recording media that are in common usage are paper and film. The paper is usually treated to make its bulk conductive and a dielectric layer of about 0.5 mil thick is coated upon its image bearing side. In its dielectric film form, a substrate such as Mylar.RTM., has a very thin conductive layer and an overcoat dielectric layer coated upon its image bearing side. Conductive side stripes pass through the dielectric layer to the conductive layer provide electrical paths to the conductive layer. In the case of paper, the potential established in the conductive layer is obtained by a combination of resistive and capacitive coupling, and in the case of film, the potential established in the conductive layer is obtained by capacitive coupling.
Conventionally, as illustrated in FIG. 1, an electrostatic image is formed upon a recording medium 10 having a thin surface dielectric layer 12 coated upon a conductive paper base material 14. The recording medium is passed between a recording head 16 and an array of complementary electrodes 18. The recording head includes an array of recording stylus electrodes 20, divided into groups, embedded in a dielectric supporting member 22. In the drawing, the complementary electrodes are in the form of backplates which conform to the contour of the recording medium for intimate contact therewith. Alternatively, they may straddle the stylus electrodes, on the same side of the recording medium. Throughout this document the term backplate will be used interchangeably with complementary electrode and it should be understood that frontplate electrodes are contemplated as well.
When the potential difference between the stylus electrodes and the recording medium conductive layer arises enough to cause the voltage in the air gap to exceed the breakdown threshold of the air, the air gap becomes ionized and air ions of the opposite sign to the potential of the conductive layer are attracted to the surface of the dielectric layer. As the dielectric surface charges up, there is a corresponding drop in voltage across the gap, so that when the voltage across the gap below the maintenance voltage of the discharge, the discharge extinguishes, leaving the dielectric surface charged. The discharge potential is established by applying a voltage of a first polarity, e.g. on the order of -300 volts, to the stylus electrodes contemporaneously with the application of a substantially equal of the opposite polarity, e.g. +300 volts, to the complementary electrodes. This causes the electrical discharge, imposing a localized negative charge to the surface of the dielectric layer 12 of the recording medium.
Typical electrographic plotters range in width from 11 inches to 44 inches, and in some cases even as wide as 72 inches, with the writing head stylus array extending across the width. Since images are usually formed at resolutions of 200 to 400 dots per inch, there are from 2000 to over 17,000 styli in a single array. Because of this very large number of styli it is not yet economically attractive to use one driver or switch per stylus. For this reason, a multiplexing arrangement is commonly used in conjunction with the discharge method described above wherein one part of the total voltage, necessary for electrographic writing, is imposed upon a stylus group and the remaining part of the necessary voltage is imposed upon its complementary electrode. The styli in the writing head array are divided into stylus electrode groups (each group being about 0.5 inch to 1.5 inches in length) so that each may consist of several hundred styli.
In order to reduce the number of drivers needed, since one driver can be used for many styli, the groups of the stylus electrodes are wired in parallel so that like styli in each group carry the same information. Then, in order to cause a selected stylus group to write, its complementary electrode is selectively pulsed. In FIG. 2 there is illustrated the conventional form for the multiplexed addressing of two sets of alternating stylus groups (referred to as As and Bs). The recording medium 10 passes between the stylus groups 20 and the backplates 18. Each commonly numbered stylus in each A-stylus group is wired in parallel with each like numbered stylus in every other A-stylus group. Similarly, all B-stylus groups are wired in parallel. Each of the stylus groups is the same length as the complementary electrode and they are offset with respect to one another so that two adjacent complementary electrodes are needed to cause a writing discharge from one stylus group. By having two complementary electrodes generally centered relative to a given selected stylus group, the voltage across the recording medium can be expected to be uniform. Although the leading and trailing stylus groups adjacent to the given selected stylus group are also influenced by an overlapping portion of the selected complementary electrodes they will not write because they are not addressed and enabled.
Generally, the firing sequence in electrographic plotters, having multiplexed stylus groups, is sequentially from one end of the writing head to the other. Such a firing sequence of the stylus groups with their associated complementary (backplate (BP)) electrodes is shown in Table 1. In associated FIG. 3 this firing sequence is diagrammatically shown in a format which will be used throughout this description. The array of rectangles 24 in the upper row represent the stylus groups, the array of rectangles 26 in the lower row represent the complementary (backplate) electrodes, and the arrows 28 indicate the firing sequence of the stylus groups.
TABLE 1 ______________________________________ Stylus Group Backplates ______________________________________ A.sub.1 BP.sub.1, BP.sub.2 B.sub.1 BP.sub.2, BP.sub.3 A.sub.2 BP.sub.3, BP.sub.4 B.sub.2 BP.sub.4, BP.sub.5 A.sub.3 BP.sub.5, BP.sub.6 B.sub.3 BP.sub.6, BP.sub.7 etc. etc. ______________________________________
It can be seen readily from Table 1 and FIG. 3 that in order for a given stylus group to write, it is necessary for a pair of overlapping backplates to be pulsed. However, because each backplate overlaps an adjacent, non-written stylus group, its pulse introduces an unwanted potential change therein immediately prior to writing by the next stylus group. For example, the portion of backplate BP.sub.2 overlying a portion of stylus group B.sub.1 introduces a potential variation, or perturbation, in the conductive layer of the recording medium in that region, immediately before stylus group B.sub.1 is to write.
Whenever the potential of a conductive layer is changed by pulsing a pair of backplate electrodes relative to the remaining backplate electrodes, which are maintained at a reference potential, the potential difference will cause current flow through it. When the pulse is extinguished, the current flows back. These is an RC time constant associated with these current flows which are the source of perturbations in the recording medium. The time scale for relaxation of the induced charges in film is on the order of tenths of milliseconds. Writing occurring upon a perturbed region of the recording medium, which perturbation has not dissipated completely, will be affected thereby and will result in visible non-uniformities in the printed information. Such image defects that were once acceptable for line and text on paper now become unacceptable as the transition is made to large solid area fill, solid modeling and full-color scanned image reproduction. This defect arising from multiplexed electrographic plotting appears as the striations (rather than uniformly printed areas) in FIG. 4. These striations are particularly observable when writing on film and are less pronounced when writing on paper.
Another cause of striations (which will not be discussed herein) is the subject of a related patent application filed contemporaneously herewith, identified by U.S. Ser. No. 07/530,719, entitled "Electrographic Marking With Dithered Stylus Group Boundaries To Eliminate Striations". It relates to the formation of objectionable striations at the electrode group boundaries, due to pulsing of the stylus electrode groups themselves.
It is the primary object of the present invention to improve the uniformity of writing upon a region of the recording medium having been perturbed by an overlapping complementary electrode.
It is another object of the present invention to substantially reduce striation defects by generating a counteracting perturbation so as to allow opposing slowly dissipating potential gradients to cancel one another.
It is yet another object of the present invention to avoid minor striations, caused by the asymmetrical sequencing of alternate stylus electrode groups by alternating the leading group in alternate scan lines.